Electron discharge apparatus



June 26, 1956 M. c. GOODALL ELEcTRoN DISCHARGE APPARATUS 2 Sheets-Sheet1 Filed April 8, 1952 /NvE/vrop MCGOODALI.

6 ZIM ATTORNEY June 26, 1956 M. c. GOODALL ELECTRON DISCHARGE APPARATUS2 Sheets-Sheet 2 Filed April 8, 1952 /NVENO/Q M. CG O O D A L L BY@W2K/5%@ ATTORNEY nited States Patent ELECTRON DISCHARGE APPARATUSMarcus Campbeil Goodaii, London, England,Y assignor to InternationalStandard `Electric Corporation, New York, N. Y., a corporation ofDelaware Application April 8, 1952, Serial No. 281,143

Claims priority, application Great Britain May 15, 1951 3 Claims. (Cl.31E-3.5)

The present invention is concerned with electron velocity modulationapparatus of the kind in which a component of electromagnetic Waves isguided along a path adjacent an electron beam so that the waves andelectrons interact with exchange of energy from one of the othercontinuously along the adjacent paths with resultant amplification ofthe waves. The invention is particularly concerned with such apparatus,referred to herein as travelling wave amplifiers, in which the electronbeam is focussed with the aid of a magnetic field.

One of the biggest difficulties with electron velocity modulationapparatus having a long electron beam path is that of focussing a highcurrent beam. More recently it has been recognized that this difficultymay be overcome by utilising stable types of electron motion which weshall refer to as Brillouin type electron flow. If electrons be injectedinto a region of crossed electric and magnetic fields, they followcurved paths. In a magnetron having an axial cathode surrounded by ananode, under the infiuence of suitable values of an axial magneticfield, and of a radial electric field between cathode and anode, theelectrons tend to follow circular paths. L. Brillouin has shown that inan axial magnetic eld and a radial electric field of given values, andin the absence of any electric field, the electrons of a space chargelimited electron beam tend to follow uniform helical paths in which eachelectron has the same axial component of velocity. In spite of mutualrepulsion, therefore, it is possible, using Brillouin type flow, foranrelectron beam to be projected through a tunnel of any desired lengthwithout spreading of the beam. u

Electron discharge apparatus has previously been proposed in whichinteraction between an electron beam having Brillouin type flow andelectromagnetic fields occur. So far as we are aware, however, suchproposals have utilised the energy associated with the axial velocity ofthe electrons. We are aware that magnetron ampliers have been propo edin which, under the action of a magnetic field, an electron beam followsa curved path lso as to interact continuously with a wave projectedalong the same path by means of a slow wave structure such as a helixabout the curved electron path. With an electron beam flowing withBrillouin type flow along a rectilinear axis it has not, so far as weare aware, hitherto" been proposed to` utilise the rotational energy ofthe beam.

According to the present/invention therel is provided an electrondischarge device comprising, in an evacuated envelope, an electron gunconstructed, ywith the aid of a substantially uniform magnetic field, toproject a beam of electrons in helical paths about a major axis of thedevice in Brillouin type fiow (as herein defined) and comprising, atleast in partwithin the said envelope, means for guiding electromagneticwaves along paths in which there is a continuous interchange between theenergy of the said waves and the rotational energy of the electron beamresulting in amplification of saidV waves. v Embodiments of theinvention will lbe described with reference to the accompanyingdrawings, in which:

ICC

Fig. l shows diagrammatically in cross-section a view of al travellingwave amplifier according to the present invention,

Fig. 2 shows the method of construction of the helix employed in Fig. 1,

Fig. 3 shows diagrammatically in cross-section a space charge waveamplifier according to the present invention, and

Fig. 4 is a section through the line IV-IV of Fig. 3.

In the embodiment of Fig. 1 reference numeral 1 indi- Cates a glassenvelope surrounding an electron `gun 2, and a helix 3. An electroncollector 4 is sealed through the other end of the tube." The electrongun 2v isespecially designed to provide Brillouin type flow through thehelix and forms part of the subject matter of my copending applicationSerial No. 281,147, filed concurrently with this application and nowabandoned. It comprises a conical cathode mounted so as to point in thedirection of the electron beam, a cathode heater 6, a first anode 7 anda second anode member S. The cathode 5 is mounted within but isinsulated from a rearward extension 9 of the first anode 7, beingcentred by means of washers 10. The first anode 7 is frusto-conical andsurrounds the emitting surface of the'cathode, being similarly orientedtherewith. The second anode member 8 is contiguous with the first anode7 and is formed as a cylinder 'whose diameter is not less than that ofthe base of the cathode cone 5. The discharge device is shown mounted ona permanent magnet 11 the ends of the device projecting through centralyapertures in pole pieces l2 and 13-respectively. ln the presence of themagnetic field between the pole pieces, electrons emitted from cathode 5are subject to radial and axial components of electric field in additionto the axial magnetic field. In consequence they rotate aroundmthecathode andare projected towards the second anode member 8. Within thesecond anode member and within the helix which may be considered asforming apart of the second anode, the electrons are subject only to theradial electric field of the anode and the axial magnetic field of thepole pieces `in addition to their own repulsive forces. The values ofthe electric and magnetic fields are chosen so that electrons movethrough thehelix with Brillouin type flow.

The helix 3 is formed from a conductor which is wound into a first helixof. small diameter and preferably of rectangular cross section., Thissmall diameter helirt is then coiled touform the larger diameter helix3. The turns of the helix 3 are located in a ceramic `spacer 14 by meansof grooves l15 (Fig. 2). The pitch of the large diameter helix is madethe same as that of the electron paths, while the smaller diameter helixis dimensioned so as to ygive rise to an electromagnetic wave componenttravelling along ltheconvolutions of the large diameter helix 3 with avelocity approximately that of the velocity of the outer electrons ofthe beam in their helical paths. The internal diameter of helix 3 issuch that, while it is as close as possible to the electron beam, theconductor of the helix does not pick up any appreciable proportion ofthe beam electrons.

The end of the helix 3 adjacent the electron gun is joined to the secondanode member by means of a rod i7 and at the other end of the helix asimilar rod 13 joins the helix to a cylinder 19 of similar dimensions tothe anode member S. Cylinder i9 may be connected vto the electroncollector 4, if desired, or these two members may be electricallyinsulated from one another.

An input wave guide 20 fitted with a matching stub 2l surrounds thedischarge device near the front end of the helix and a similar waveguide 22 and matching stub 23 provides an loutput feed for the electroncollector end of the helix.

The rods 17 and 1S act as wave guide probes in the manner of similarrods employed in conventional single helix type travelling wave tubes.The anode member 8 and the cylinder 19 co-operate with anges 24 and 25respectively to form quarter-wave sleeves permitting passage of theenvelope 1 through the outer walls of the wave guides in the same way asin the conventional single helix type of travelling wave tube.

In operation the cathode heater 6 is connected to appropriate ones ofpins 26 sealed through the base of the envelope 1 to a current supplyrepresented in Fig. l by the battery 27. in Fig. l the cathode is shownconnected to one end of the heater and the iirst anode 7 is connected tothe positive pole ot a supply 2S, shown as a battery in series with 27.With this arrangement and a magnet capable of providing an axial fieldof the order of 600 gauss, the helix .3 may be some 3 cms. long while anelectron beam of some 90 milliamperes may be obtained from the electrongun 2 when connected to a 300 v. anode supply.

As the electrons are travelling with Brillouin type flow, their axialvelocities will all be the same. ln consequence the anode voltageapplied to the tirst anode 7 may be adjusted so that they all executehelices of the Same pitch as the double helix 3. Since the cathode 5 isycontinued substantially to a point, the electron beam tends to rotate asa whole with all electrons having the same angular velocity, theoutermost ones travelling with approximately the same velocity in theirpaths as the slow component of the electromagnetic waves guided by thedouble helix 3. In consequence these waves are amplified continuouslyduring their path along the helix. Since both the electrons and thewaves with which they interact travel over paths which may be severalwavelengths long for a small axial flow, it will be appreciated that atube according to the present invention may be made much smaller thanthe conventional travelling wave tube while requiring much lowervoltages for its power supply.

The present invention is particularly applicable to space charge waveamplifiers. In the space charge wave amplifier, instead of a slow waveguide structure such as a helix being employed, use is niade of thespace charge in an electron beam to guide electromagnetic waves, whichare then amplified by interaction with either a second beam or withanother part of the same beam travelling at a slightly differentvelocity. Most of the prior proposals for space charge wave amplifiershave utilised two distinct electron beams originating from separateelectron guns` It has, however, been recognised that amplitication maytake place in a single electron beam where different layers of electronsmove with slightly ditierent velocities. if we liken an electron beam toa fluid, using the hydrodynamic concept we may say that ampliticationwill occur if there is any sheer in the beam. Such sheer will occur inBrillouin type flow with an annular electron beam, the electrons on theinner circumference of the aiinulus having negligible angular velocitywhile those on the outside have a velocity suicient to maintain them ina constant diameter path in the presence of the existing superimposedelectric and magnetic fields. In such a beam there is evidentlytangential sheer at all radii between the inner and outer radii of thebeam. This type of flow may readily be produced by modifying theelectron gun of Fig. l so that the cathode, instead of tapering to apoint, isshaped as the frustum of a cone.

A space charge wave amplifier according to the present invention isshown in Fig. 3. While the discharge device 29 of Fig. 3 differs fromthat of Fig. l the remaining arrangements are very similar andcomponents having similar functions in Fig. 3 to those of Fig. l bearthe same reference numerals. The electron gun 30 of Fig. 3 has afrusto-conical cathode 31 orientated with its smaller diameter endfacing the direction of propagation of the electron beam. Cathode 31 issurrounded by a similarly orientated frusto-conical rst anode 32. Thecathode 31 is mounted in similar manner to the cathode 5 of Fig. l`

by means of washers 1t) from a rearward extension 9 off the first anode32. First anode 32 is joined by a rod 33. parallel to the axis of thedischarge device to a pick upy helix 34. A wave guide 20 having amatching stub 21 surrounds the helix 34, but in the present embodimentthe inner member of the quarter wave sleeve in the rear wall of waveguide 20 is provided by the anode extension 9 instead of by the cylinder8 of Fig. l.

At the other end of the tube a helix 35 similar to helix 34 is providedwithin the wave guide 22 and is joined toi the inner member i9 of theoutput quarter wave sleeve: by the rod 36.

The helices 34 and 35 are each constructed by forming. a conductor intoa helix of square cross section and forming this into a single turn coilin a plane at right. angles to the electron beam; the arrangement isshown more clearly in Fig. 4. The helices 34 and 35 are joinedelectrically by means of a coating 37, such as graphite, on the innerenvelope Wall in the region between them.. As is seen in Fig. 4, thehelices do not form continuous` turns; the coating 37 is arranged toproject under the end turn of the helix but not under other turns, as isiti-- dicated at 38, Fig. 4. ln this way the second anode for' theelectron gun 3i) is provided by the helices 34 and 35v together with thecoating 37 joining them. The electron beam from the gun is adjusted soas not to impinge upon the helices 34 and 3S but just to graze them. Thecoil 34 together with the rod 33 picks up energy from the waveguide 20and launches a Wave upon the electron beamhaving a component travellingalong the coiled axis of helices 34 and 35, at approximately the samevelocity as that of the outermost electrons. The space charge off thebeam guides the electromagnetic waves from helix 34- towards helix 35,and is amplified by interaction with the inner beam electrons. Asexplained above, due to the beam being annular, at different radii theelectrons are: moving with different angular velocities, the innermostelectrons being at cathode potential. Amplification takes place insimilar manner to that of the double beam amplitiers of the prior art,except that the interacting electrons and waves are revolving together,the amplified space charge wave being picked up by the helix 35 andlaunched into the Wave guide 22.

While the principles of the invention have been dcscribed above inconnection with specic embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way ot example and not as a limitation on the scope ot theinvention.

What we claim is:

l. A travelling wave amplifier comprising in an evacuated envelope meansfor producing, along a given rectilinear path, solely two steady lieldsconsisting of an axial magnetic field and a radial electrical field,means for projecting into said path, with a finite velocity in the axialdirection, an electron space charge rotating about said axis to therebyproduce a beam in which the electrons travel in uniform helices and havethe same axial velocity and means for guiding electromagnetic Wavesalong a path having a component of its length substantially parallel tothe rotational components ot the beam and a velocity substantially thesame as said rotational components to provide continuous interchangebetween the energy of said waves and the rotational energy of the beam,resulting in amplication of said waves, said wave guiding means being aconductor coiled into a rst helix which is itself coiled into a furtherhelix of larger dianv eter, the arrangement being such that the pitch ofthe larger diameter helix is substantially the same as that of theelectron paths and the winding of the smaller dianv eter helix is suchthat a component of the said electromagnetic waves travels along theconvolutioiis of the larger diameter helix with a velocity substantiallyequal to that of the outer electrons along their helical paths,

said electron projecting means comprising a substantially conicalcathode with its apex pointing in the direction of propagation of theelectron beam, a frusto-conical first anode surrounding the said cathodeand similarly orientated a cylindrical second anode member contiguouswith the first anode and a collector electrode facing the cathode at theother end of the said envelope.

2. A travelling wave amplifier according to claim 1 comprisingrespective hollow Wave guides coupled to the ends of the said helicalconductor, the said device being mounted to pass through the waveguides, annular flanges on the wave guides and cylindrical memberswithin the discharging device co-operating to form towards either end ofthe device respective quarter wave sleeves for passage of the envelopethrough the end wave guide walls and a conducting rod forming a pick-upprobe in the wave guide at either end of the helix joining the end ofthe helix to the adjacent said cylindrical member at the electron gunend of the said device forming the said cylindrical second anode member.

3. A travelling wave ampliiier according to claim 1 in which the saiddischarge device is mounted on a permanent magnet having annular polepieces receiving the respective ends of the said envelope.

References Cited in the file of this patent UNITED STATES PATENTS2,241,976 Blewett et al May 13, 1941 2,295,315 Wol Sept. 8, 19422,329,118 Inman Sept. 7, 1943 2,424,965 Brillouin Aug. 5, 1947 2,579,654Derby Dec. 25, 1951 2,610,308 Touraton et al Sept. 9, 1952

